The present invention relates in general to input buffers for semiconductor integrated circuits and, more particularly, to methods and apparatus for protecting input buffers from high voltages which are applied to integrated circuits for purposes such as testing and activating programmable interconnection elements. While the present invention can be utilized in a variety of fabrication techniques it will be described with reference to input buffers made using metal oxide silicon (MOS) fabrication techniques and, in particular, complementary MOS (CMOS) fabrication techniques for which it is particularly applicable and initially being applied.
Integrated circuits are tested to ensure that they are free of defects and that the circuits function properly and according to specifications. Testing includes "burn-in" where the circuits are subjected to extremes of operating temperatures and voltages to identify infant failures. Alternate testing can be performed on integrated circuits to test application specific features. Since alternate testing is normally performed by the manufacturer and is not intended to be used by purchasers of the integrated circuits, an electronic key is typically provided to enable the manufacturer to place the circuits into a test mode. A common key is a voltage which is a predetermined level above the maximum specified supply voltage for the circuits. Thus a high voltage key, sometimes referred to as a "supervoltage", may be applied to an input pin of an integrated circuit to place the circuit in a test mode. Once in the test mode, test vectors can be entered using other pins of the integrated circuit to perform required tests.
In some integrated circuits, defects can be corrected by means of one time programmable (OTP) permanent electrical connections which can be implemented with an antifuse. In dynamic random access memories (DRAMs), failing memory cells, rows or columns which are detected during testing can be remapped to functional redundant memory cells, rows or columns by selective permanent programming of antifuse elements. OTP connections are also used to permanently store data on integrated circuits such as programmable logic arrays (PALs), programmable logic devices and programmable read only memories (PROMs) and for other integrated circuit applications. An antifuse element is programmed by applying a programming voltage ranging from approximately 9 volts to approximately 13 volts which is above the maximum supply voltage for the integrated circuits.
In at least both these instances, testing using a supervoltage key and programming antifuses or similar devices, a high voltage is applied to an input pin of an integrated circuit. Portions of an integrated circuit which receive the supervoltage or the programming voltage are designed to withstand the high voltages at least for the time required to perform these operations. Unfortunately, these portions of the integrated circuits cannot be severed practically from input buffers of the integrated circuits which operate with normal signals levels and can be damaged by the high voltage applied for testing and programming. Even if the applied high voltage is less than the breakdown voltage of the devices making up the input buffer such that immediate damage is not done, the resulting voltage stress over time can result in permanent damage to the input buffer and failure of the integrated circuit.
Accordingly, there is a need for improved input buffers which are protected from damage due to high voltage inputs and methods of protecting input buffers.